Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
A flash memory is a type of memory that can be erased and reprogrammed in blocks instead of one byte at a time. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. Each of the cells can be electrically programmed on a random basis by charging the floating gate. The data in a cell is determined by the presence or absence of the charge in the floating gate. The charge can be removed from the floating gate by a block erase operation.
As electronic devices become smaller, their components such as the flash memory must also be reduced in size. At the same time, the memory devices are becoming denser to keep up with the increasing performance of the electronic devices.
As memory devices are scaled, the tolerances in the manufacturing are reduced as well. This translates into a greater quantity of defects from shorting of different layers of the memory devices, thus reducing the yield and increasing the cost of the device. Since one reason for the smaller, denser memory devices is to reduce costs, the decreased yield problem is contrary to the move to more advanced processes to lower costs.
Redundancy has been used to improve the defective memory situation. For example, some memory devices have column and memory block redundancy that can be used to replace defective columns and blocks with extra columns and/or blocks of memory. NAND flash memory typically uses block redundancy to replace defective memory blocks. However, only a limited quantity of redundant blocks can be included on a memory die. Remapping an entire memory block when only one row or column is defective in a memory block can eventually use up all of the redundant memory blocks. This can leave too many defects on the die.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a flash memory device that has improved handling of defective memory blocks.